The present invention relates to an ultrasonic nondestructive test apparatus which detects changes in the velocity of the ultrasonic wave propagating in an object to be tested by interferometry.
The ultrasonic measurement method which utilizes the principle of interference is conventionally known as a method for measuring the velocity of sound in a liquid or a gas with precision. The method for causing interference is roughly classified into the method for varying the frequency of the ultrasonic waves and the method for varying the length of a propagation path of ultrasonic waves. An ultrasonic nondestructive test apparatus is known which utilizes this method for varying the length of the propagation path of ultrasonic waves. In this apparatus, an ultrasonic transmitter-receiver is arranged at the bottom of a vessel. A continuous electric signal generated by a high-frequency oscillator having an oscillating frequency of f is modulated into a high-frequency pulse signal by a pulse modulator. The high-frequency pulse signal thus obtained is applied to the transmitter-receiver in order to generate pulsed ultrasonic waves into a sample liquid as an object to be tested held in the vessel. The pulsed ultrasonic waves are reflected by a reflecting surface or a liquid surface of the sample liquid which is arranged in opposition to and parallel to the transmitter-receiver and are then returned to the transmitter-receiver.
In the ultrasonic nondestructive test apparatus as described above, the sample liquid is gradually discharged to gradually lower the liquid surface of the sample liquid, so that the length of the propagation path changes and maximum and minimum values of the amplitude of the reflected pulsed ultrasonic waves are obtained due to the interference. Accordingly, if the changes in the length of the propagation path within the sample liquid are accurately measured, the velocity of sound in the sample liquid may be determined since the oscillation frequency f of the oscillator is given.
It is also possible to measure the velocity of ultrasonic waves propagating in the liquid by arranging a pair of a transmitter and a receiver with a distance therebetween in a sample liquid held in a vessel and varying the distance between the transmitter and the receiver.
However, in the conventional apparatus as described above, the changes in the distance between the transmitter-receiver and the sample liquid or between the transmitter and the receiver, that is, the changes in the length of the propagation path of the sound waves are small. Therefore, precise measurement of such small changes requires, skills and a long measurement time. Furthermore, since the ultrasonic transmitter-receiver generates plane waves, measurements of two-dimensional distribution of the velocity of sound in an object to be measured may not be performed.